The present invention relates to a multi particle beam lithography system, a sensor therefore and a method. Such lithography systems, alternatively denoted litho system, generally operate according to a method for transferring a pattern onto the surface of a target, thereby normally using a so called particle beam tool for generating said multitude of charged particle beamlets, which beams may be scanned in one or more directions by means of electronic controls. The multitude of beamlets, also denoted writing beams in the following is to be calibrated by means of a sensor. A method upon which such litho system is commonly based comprises the steps of generating a plurality of writing beams for writing said pattern on said target surface, usually a wafer or a mask. Preferably a writing beam is constituted by an electron beam, emitted by a writing beam source, which may e.g. comprise a cathode, and which may be supplemented with writing beam shaping means such as an array of apertures for converting a beam emitted by said source into a massive plurality of significantly smaller diameter. Also, such known litho system may be provided with collimating means for directing a source beam or a set of generated writing beams into parallel.
In such known method each writing beam is deflected separately at writing said pattern on to said target surface, for interrupting said writing process. This is performed by means of e.g. an array of electrostatic deflectors and beamlet blankers through which the writing beams are passed within the system. Especially in case of a massively multi writing beam system as according to the present invention, such deflectors are provided with so called modulation information by a signalling means. Such part of a lithography system for writing patterns onto wafers or masks, is in the following denoted a beam tool. Such beam tool and such lithography system can in more detail be known from e.g. patent publication WO2004038509 in the name of Applicant.
Further typifying the electron beam lithography under consideration, application thereof is directed to high-resolution purposes. Nowadays applications are capable of imaging patterns with a critical dimension of well below 100 nm feature sizes. Multi beam in this respect in particular relates to so-called massive multi beam system comprising e.g. a number of writing beams in the order of 10000 and higher. In this respect a typical application as currently offered by Applicant comprises 13.000 writing beams. Future developments however are focussed to litho tools comprising a number of beams in the order of 1 million, which systems are intended to utilise a principally same kind of sensor.
Such exposure lithography systems only become commercially viable when at least the position of all of the electron beams is precisely controlled. Due to various circumstances such as manufacturing tolerances and thermal drift, a beam generated in the writing beam tool of a lithography system is however likely to have one or more errors which render it invalid for writing. Such error may be a positioning error, with respect to a designed grid. Such erroneous feature of a beam tool, and therewith of the lithography system, severely affects the quality of the pattern to be written. Yet, the position of an e-beam near the surface to be exposed is required to be known within a distance of a few nanometers and should be able to be calibrated. In known litho systems this knowledge is established by frequent calibration of the beam position.
Apart from above mentioned specific feature, also other features of a writing beam are desired to be known accurately and preferably at multiple instances, and therefore swiftly, during operation of the beam tool, in particular during writing of a wafer, so as to allow an early adaptation of the writing process of a wafer, and to thereby increase the number and chance of correctly written wafers or fields thereof.
Known calibration methods commonly comprise at least three steps: a measuring step wherein the position of the electron beam is measured, a calculating step wherein the measured position of the electron beam is compared to the desired position of that beam, and a compensation step wherein the difference between the measured position and the desired position is compensated for, either in the software or in the hardware of the lithography system or said electron beam tool thereof.
Such known measuring or calibration systems hardly pose any inhibit to electron beam lithography tools which are characterised by a relatively low throughput, e.g. only part of a wafer is patterned within one hour, or by a relatively limited number of writing beams as compared to a massively multi beam system. For maskless systems directed at high throughput or with a massively multi beam system as is focused on by the present invention, the known calibration systems form a limiting factor to the desired high capacity and high throughput, maskless lithography system.
With the known methods, a charged particle beam system, e.g. an electron beam based system, needs to be calibrated a large number of times. Where this may be acceptable for a single beam litho system or with a number of beams, this circumstance becomes a problem if 13000 or more beams are to be calibrated in series. In such case the time needed for calibration would far outweigh the time needed for actually treating a field on a wafer. Therefore, so as to increase the throughput of the known litho system, and in accordance with an idea underlying the present invention, the calibration procedure should be speeded up significantly.
In the art, several calibration methods for electron beam lithography systems are known. Most use marks residing in either the wafer stage or the wafer or in both. A sensor then performs e.g. the detection or the position of a beam. The sensor being a charged particle sensor, measures the amount of secondary or reflected electrons created by the marker.
One example of a method using charged particle sensors in combination with a plurality of charged particle beams is provided by the U.S. Pat. No. 5,929,454. It discloses a method to detect the position of a plurality of electron beams by using marks positioned on the wafer or stage. The mark is a parallel line pattern and used for several measurements. All measurements are performed by detection of either secondary or reflected electrons from said alignment mark upon scanning. The position of the alignment mark is determined on the basis of the displacement amount of the electron beams and the detection result. Such an electron detector has the advantage of rapidly determining any primary or secondary electrons, however is relatively bulky, i.e. measures within a range of mm, and thereby not suited for litho systems utilising a massive multiplicity of charged particle beams, e.g. 13000 beams or more. In such latter kind of litho systems a typical pitch between beams is in the order of tenths of mm, e.g. typically 150 μm in a present day 13.000 beam system. Apart from the above volume feature of the known sensor and calibrating system, the known system is also relatively expensive, and moreover not capable of calibrating a massive amount of charged particle beams in qualitative and sufficiently swift manner.
In a massively multi-electron-beam lithography system also other problems arise, in that e.g. adjacent beams should not influence the accuracy of the position detection. Also, it is not clear with the known method and system how to perform both data acquisition and data processing within a reasonable limit of time for all of the massive amount of writing beams, i.e. within a period of time which is much less than the period required for writing a wafer. The latter problem is especially significant because of an additional requirement, at least strongly desired feature common in the art, of frequent calibration of the entire beam tool during the process of writing a wafer, so as to monitor and timely compensate for e.g. said earlier indicated dynamic drift of writing beams. Such manner of proceed prevents undue loss of treated wafers, i.e. of performed work by an expensive machine.